Electrodeposition of rhodium

ABSTRACT

A technique is described for electroplating rhodium utilizing a soluble rhodium anode. The technique involves maintaining a constant rhodium concentration in the electrolyte by continuous dissolution of a rhodium anode, such dissolution being effected by imposing a pulsed signal on the anode.

United States Patent [191 [111 3, 7 5,267

Yahalom Nov. 2'7, 1973 [54] ELECTRODEPOSITION OF RHODIUM 3,276,976 10/1966 Juliard 204 47 x [75] Inventor: Joseph Yahalom, Haifa, Israel [73] Assignee: Bell Telephone Laboratories, Primary Examiner-G Kaplan Incorporated, Murray Hill, NJ. y Keefauver et [22] Filed: Jan. 4, 1973 21 1 App] 3214,26 57 ABSTRACT [52] US. Cl 204/47, 204/DlG. 8 A t ghnique is described for electroplating rhodium utilizing a soluble rhodium anode The technique in- Fleld of Search volves maintaining a constant rhodium concentration 204/111 in the electrolyte by continuous dissolution of a rhodium anode, such dissolution being effected by impos- [56] Ref e s C t ing a pulsed signal on the anode.

UNITED STATES PATENTS 2,951,978 9/1960 Dickson et al 204/47 X 3 Claims, 1 Drawing Figure POTENTIOSTAT I ELECTRODEPOSITION OF RHODIUM This invention relates to a technique for electrodeposition of rhodium. More particularly, the present invention relates to a technique for electro-deposition of 5 rhodium utilizing a soluble anode.

A wide variety of processes have been described in the literature for electrodeposition of precious metals, particularly platinum and palladium. In the operation of such processes, it has been conventional to add a salt of the precious metal to the plating bath from time to time as the precious metal content becomes depleted, thereby maintaining themetal content at the required concentration. In order to obviate this economically burdensome and time consuming operation, workers in the art proposed the use of soluble precious metal anodes in conjunction with an electrolyte capable of dissolving the metal anode at a rate substantially equal to that at which the metal is deposited cathodically upon articles immersed in the electrolyte. Unfortunately, in the case of rhodium, the use of soluble anodes has not been practicable due to the passivity of rhodium in the plating bath.

In accordance with the present invention, this prior art deficiency has been successfully obviated by a novel procedure utilizing a soluble rhodium anode. Briefly, the inventive technique involves maintaining a constant rhodiumconcentration in the electrolyte by continuous dissolution of a rhodium anode. This end is attained by applying a potentialdifference between a soluble rhodium anode and an auxiliary electrode so as to develop a potential on the rhodium anode with respect to a reference electrode which varies periodically between an upper and lower value. Under these conditions, a continuous breakdown of the anode passivation occurs and dissolution results at a rate controlled by pulse height and duration.

The invention will be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

The FIGURE is a schematic representation of a typical apparatus utilized in the practice of the present invention.

With reference now more particularly to the FIG- URE, there is shown a plating vessel 1 1 having disposed therein a soluble rhodium anode 12, a cathode 13 which comprises the article to be plated and an auxiliary electrode 14 comprising graphite or any conductive material which is inert to chemical attack by an electrolyte 15. Electrolyte 15 is an acid having a pH less than one selected from among hydrochloric, sulphuric, phosphoric, perchloric acids and the like. The system also includes a conventional reference electrode 16, typically a calomel electrode, which serves to maintain the article to be plated at a controlled potential with respect to the electrolyte. The auxiliary electrode 14 provides current to maintain the anode at a controlled potential. Each of the electrodes is shown connected to a pulsing means 17, typically a potentiostat including a pulse generator. The anode and cathode are also shown connected to a direct current source 18 which is capable of delivering between 2 and 4 volts to the cathode and anode.

In the operation of the process, a dc. bias is established between the rhodium anode and the article to be plated. Following, a difference of potential is impressed between the soluble rhodium anode and the auxiliary electrode so as to develop a potential on the rhodium anode with respect to the reference electrode which varies periodically between an upper and lower value. In the practice of the present invention, this potential may vary from 950-1450 mv (hydrogen scale) at the upper value and from 550 to 50 mV (hydrogen scale at the lower value. These ranges are determined by consideration of plating rate and the rate of redeposition of rhodium upon the soluble anode. The period of time that the sample is maintained at either the upper or lower potential may conveniently range from 1 milliseconds, the shorter time period corresponding with higher plating rates and the converse. It will be appreciated by those skilled in the art that the lower limit is dictated solely by instrument limitations whereas the upper limit is dictated by practical considerations relating to the plating rate.

An example of the present invention is described below. The example is included merely to aid in the understanding of the invention and variations may be made by one skilled in the art wihtout departing from the spirit and scope of the invention.

EXAMPLE 1 A platinum wire, 20 mils in diameter, was selected for plating with rhodium. The wire was immersed in an electrolyte comprising a 50 percent sulphuric acid solution having a pH of approximately one in an apparatus of the type depicted in the FIGURE. The auxiliary electrode was a graphite rod and the reference electrode a calomel electrode. A dc. bias of 4 volts was impressed between a rhodium anode, obtained from commercial sources, and the platinum wire. Plating was then effected by establishing a potential of 50 mV on the soluble rhodium anode with respect to the reference electrode for a duration of 1 millisecond and then establishing a potential of 1,100 mV for a duration of 1 millisecond. This procedure was continued successively for 15 minutes, so resulting in the deposition of a uniform coating of rhodium on the platinum wire. The apparatus employed was initially calibrated so that the role of rhodium dissolution was equal to the rate of rhodium deposition.

What is claimed is:

1. Technique for the electrodeposition of rhodium which comprises the steps of (a) immersing a soluble rhodium anode, a cathode comprising the article to be plated, an auxiliary electrode and a reference electrode in a plating both comprising an acid having a pH less than one (b) applying a direct current to anode and cathode and (c) establishing a difference potential between said anode and the auxiliary electrode so as to develop a potential on the anode with respect to the reference electrode which varies periodically between an upper value ranging from 950-1 ,450 mV (on the hydrogen scale) and a lower value ranging from 500 to 50 mV (on the hydrogen scale), so resulting in the electrodeposition of rhodium upon the article to be plated, the plating bath dissolving the rhodium anode at a rate equal to the rate of deposition.

2. Technique in accordance with claim 1 wherein said potential varies at a periodicity from 1 to 100 milliseconds.

3. Technique in accordance with claim 1 wherein said acid is sulphuric acid. 

2. Technique in accordance with claim 1 wherein said potential varies at a periodicity from 1 to 100 milliseconds.
 3. Technique in accordance with claim 1 wherein said acid is sulphuric acid. 